The present invention relates to a clock generating apparatus and a frequency calibrating method of the clock generating apparatus, and more particularly to a phase-locked loop applied in a multi-standard communication system having a short frequency calibrating time, and a method thereof.
In a wireless communication system, a phase-locked loop is utilized to generate a clock signal having a predetermined oscillating frequency to accommodate a specific communications standard. The phase-locked loop is designed to be able to generate different oscillating frequencies corresponding to different communications standards respectively. The frequency tuning range of an oscillator in the phase-locked loop should therefore be wide enough to accommodate the multi-communications standards. Conventionally, when a wireless communication system is to be operated under a specific communications standard, the oscillator is first controlled to search over the whole usable frequency range in order to calibrate the oscillator for outputting a target frequency corresponding to the specific communications standard. The conventional communication system may require a long time to search the target frequency when more and more communications standards are embedded into one wireless communication system. Therefore, speeding up the calibrating time of a phase-locked loop to generate the target frequency in a multi-standard communications system has become an important issue in the field.